The faint young Sun
The Sun shone 30% weaker — yet the ocean did not freeze
See it on the timeline →In depth
The paradox was formulated by Carl Sagan and George Mullen in 1972. Stellar models are unambiguous that the Sun was around 70 per cent as luminous, while the geology is equally unambiguous that liquid water existed. Proposed resolutions include high levels of CO₂ and methane, lower albedo from less exposed continent and fewer clouds, and a contribution from a denser nitrogen atmosphere. The point is not merely historical: it is the same climate physics that makes greenhouse gases set the temperature today, tested against a radically different state of the planet.
For children
When life began, the Sun gave off about a third less heat than today. Do the arithmetic and Earth should have been a block of ice. But we know the ocean was liquid — so something must have held the heat in. The answer is greenhouse gases in the air, far stronger than today's.
Connects to
Part of Terra Saga — the story of Earth on a timeline. terrasaga.org